Commercial preparation of many different food and non-food products requires the use of pumping equipment suited for pumping materials which do not readily flow or are otherwise difficult to pump. For example, food products such as cheeses, lards and shortenings, ground meat, sugars, and doughs typically require the use of specialized, multi-stage pumping devices for effecting desired movement of such materials. Similarly, non-food products such as adhesives, soaps, putties and caulking compounds, and the like require the use of specialized pumping equipment if efficient handling is to be achieved.
A pumping apparatus is disclosed in U.S. Pat. No. 4,792,294. This apparatus has been found to effectively pump products which do not readily flow and for which the pump is particularly well suited. The pumping apparatus includes a pair of cooperating, counter-rotating screw augers within a housing. Each of the screw augers includes helical flights intermeshed with the flights of the other auger, and the flights are configured to provide a two-stage pumping action--namely, (1) a first, upstream, non-positive displacement pumping, and (2) a second, downstream, positive displacement pumping.
Although the pump disclosed in U.S. Pat. No. 4,792,294 is particularly effective in pumping materials which do not readily flow, it has been unexpectedly discovered that some such materials, while being effectively pumped per se, can nevertheless, under some conditions, undergo a change in character, consistency, or other material properties. With some products, significant changes in properties may be characterized as a loss of product integrity which is of such a nature as to render the product commercially unacceptable.
For example, the pump described in U.S. Pat. No. 4,792,294 employs the intermeshed helical flights of the two augers in the non-positive displacement stage to feed the material downstream to the intermeshed helical flights of the positive displacement pumping stage. With some materials and/or by intentional pump design, there may be a tendency in some situations for the material to be pumped at a higher flow rate through the non-positive displacement first stage than through the positive displacement pump second stage.
Since the positive displacement second stage can only handle a generally constant flow rate at any set speed of pump operation, the tendency of the non-positive displacement first stage to produce a higher flow rate results in the material "slipping" relative to the non-positive displacement first stage helical flights. That is, since not all of the flow rate of material which could be pumped by the first stage can be handled by the second stage, some of that material in the first stage must remain in the first stage while the first stage helical flights rotate against and past that material. This can result in shear forces being applied to the material as the material "slips" relative to the first stage helical flights or "slips" relative to some of the other material that is being carried with, and pumped by, the first stage helical flight.
The shear forces imposed upon portions of the material in the first stage as a result of the above-described "slip" may result in excessive shear strain in the material that can change the character of the material. This excessive shear strain may also be characterized as excessive "working" of the material such that the material may lose its desired characteristics or integrity.
Italian sausage meat is one food product material which is sensitive to the imposition of shear forces. Italian sausage meat contains a significant quantity of fat compared to other meat products, including even other types of sausage meat. If the Italian sausage meat is subjected to excessive shear forces and resulting strain, then the sausage meat will appear to be smeared with fat all over the exposed surfaces. The desired visual demarcation or boundaries between the lean meat and fat is thus undesirably eliminated. Such a condition is commercially less acceptable.
Of course, it is to be realized that the advantages described for the pumping apparatus in U.S. Pat. No. 4,792,294 still exist with respect to the actual pumping per se of material regardless of the fact that the material character may change. In particular, even though the material characteristics may change owing to slippage in the first stage of a pumping apparatus of the type described in U.S. Pat. No. 4,792,294, that pumping apparatus nevertheless functions as designed for such material to provide the desired non-positive displacement pumping action to continuously feed the positive displacement downstream stage so as to avoid cavitation. By providing the first and second pumping stages in immediate succession, a pressure drop at the transition is desirably avoided.
Further, the arrangement of the pumping apparatus desirably promotes "piercing" of the material being fed by gravity into the pumping apparatus so as to avoid the problems of "tunneling" and "bridging" which had been heretofore common with other pumping apparatus constructions.
The pumping apparatus is also effective to pump such materials at the desired flow rate of the positive displacement pumping stage--regardless of slippage in the non-positive displacement pumping stage--in a manner that permits driving the pump from a drive mechanism at the downstream (outlet) end. This permits the use of positive pressure dynamic seals at drive shafts at the downstream end where the pumping apparatus is subjected to positive pressure so that sealing is more easily achieved in comparison to sealing around drive shafts at the pumping apparatus upstream end which may be subject to vacuum from an upstream processing stream, such as sometimes occurs in the processing of certain cheeses, for example.
Nevertheless, even though difficultly flowable materials can be advantageously pumped by the pumping apparatus described in U.S. Pat. No. 4,792,294, it would be desirable to provide an improved pumping apparatus for use with materials that are particularly shear sensitive so as to eliminate, or at least substantially reduce, the application of excessive shear strain on the material which might cause it to change its material characteristics.
It would be advantageous to provide such an improved apparatus for pumping, without excessive shear strain, shear sensitive materials that may be characterized as thixotropic or non-newtonian.
Further, it would be desirable to provide an improved pumping apparatus in which a more gentle, non-positive pumping action is provided and in which some sort of inherent adjustability may be incorporated into the non-positive displacement pumping action.
It would be especially beneficial if such an improved pumping apparatus could accommodate, in a manner that reduces imposition of shear forces on the material, a tendency of the first stage to effect a higher flow rate than can be handled by the second stage.
Further, it would be desirable to provide an improved pumping apparatus in which the inlet geometry could be adapted to reduce the shear forces applied to the material.
In applications where the pumping apparatus is connected directly to the outlet of equipment maintained under vacuum, such as a continuous evaporator, it would be desirable to provide an improved pumping apparatus for creating and maintaining a net positive suction pressure at the positive displacement stage of the pumping apparatus.
Finally, it would be desirable, especially in food product applications, to provide an improved pumping apparatus for effecting a sanitary pumping operation. In particular, it would be desirable to provide, to the extent possible, a "first in-first out" pumping operation, especially with food materials having characteristics sensitive to imposed shear forces.